Second air traffic fault this month disrupts Scotland flights

NATS has fixed a network fault at its Prestwick centre in Scotland that forced air traffic restrictions and left airlines with delays on 21 September, the UK’s air traffic control provider has said.

The company said it experienced the issue early that morning, and that air traffic regulations were applied to manage traffic demand while its engineers investigated, in order to maintain safety.

“The issue has now been fixed and we are running at full capacity,” NATS said, adding that it was working with its customers “to recover their residual delays and help get their operations back to normal”.

NATS said the fault was “nothing to do with the problem we had earlier this month”.

What the 8 September report found

That earlier failure, on 8 September, led to a preliminary report which NATS published on 18 September at the request of Heidi Alexander, Secretary of State for Transport, with a full report to follow.

The report said the incident was caused by a software defect in a small part of the National Airspace System (NAS), which underpins the management of UK airspace. The software allocates codes that are used to identify individual aircraft on radar.

A request was paused when the system received a message for a higher priority activity, NATS said, and when processing resumed “it did not resume correctly and the resulting output was corrupted”, affecting some flight data updates. That happened “in the space of a millisecond”.

Although the software issue affected only flights handled by the London Area Control centre – higher level flights operating mainly above 24,500ft – restrictions had to be applied across the UK so that the system could be restarted and flight data reloaded, the report said.

Restrictions were in place for some six hours. Operations returned to normal the same evening, but it took more than two days for the backlog to clear, with hundreds of thousands of passengers’ travel plans disrupted, according to NATS.

Martin Rolfe, NATS chief executive, said the failure was unrelated to the company’s major system outage in August 2023 and dismissed speculation that it had been caused by military intervention. “This was a software issue in a specific part of our flight data system, that we have traced to a small subsection of coding,” Rolfe said, adding that mitigation was in place “while a permanent fix is safety tested and deployed”.

Rolfe also said: “I would like to apologise again, very sincerely, to everyone who was affected”, adding that “at no point last week was safety in question”.

NATS said it had invested “well over one billion pounds” in systems and technology over the past 10 years, and would soon submit plans to the Civil Aviation Authority to invest a further billion pounds by the end of 2033.